Lipids
Lipids, Lipoproteins, and Dyslipidemia
Cholesterol and triglycerides travel through plasma in lipoprotein particles. The particle’s lipid content, apolipoproteins, and metabolic route determine what a lipid assay measures and how to interpret an abnormal result.
Lipid molecules
Fatty acids are hydrocarbon chains with a terminal carboxyl group. Saturated fatty acids contain no carbon-carbon double bonds. Most naturally occurring unsaturated fatty acids contain one or more cis double bonds that bend the chain and reduce packing. Industrial hydrogenation can produce trans double bonds. Linoleic acid, an omega-6 fatty acid, and alpha-linolenic acid, an omega-3 fatty acid, are essential because humans cannot synthesize them. Twenty-carbon omega-6 and omega-3 fatty acids provide precursors for prostaglandins, thromboxanes, prostacyclins, and leukotrienes.
Triglycerides contain three fatty acids esterified to glycerol. They are neutral, hydrophobic storage lipids and form much of the core of chylomicrons and very-low-density lipoproteins (VLDL).
Glycerophospholipids contain two fatty acids and a phosphorylated head group attached to glycerol. Their hydrophobic tails and hydrophilic heads make them amphipathic. They form cell-membrane bilayers and the surface monolayer of lipoproteins.
Cholesterol has four fused rings, a hydrocarbon tail, and one hydroxyl group. The hydroxyl faces the aqueous surface of a membrane or lipoprotein. Esterification of the hydroxyl with a fatty acid forms cholesteryl ester, which moves into the hydrophobic particle core. HMG-CoA reductase controls a major rate-limiting step in cholesterol synthesis. Cholesterol supplies cell membranes and is a precursor for bile acids, steroid hormones, and vitamin D. The body eliminates cholesterol mainly through conversion to bile acids and biliary secretion.1
Lipoprotein particles
A lipoprotein has a core of triglyceride and cholesteryl ester surrounded by free cholesterol, phospholipid, and apolipoproteins. A larger lipid-to-protein ratio produces a larger, less dense particle. Ultracentrifugal density defines the traditional particle classes.2
| Particle | Density (g/mL) | Approximate diameter | Major lipid | Structural apolipoprotein | Main laboratory meaning |
|---|---|---|---|---|---|
| Chylomicron | <0.930 | 75–1,200 nm | Triglyceride | apoB-48 | Carries dietary lipid; produces a cream layer after refrigerated standing when present in excess |
| Chylomicron remnant | 0.930–1.006 | 30–80 nm | Triglyceride and cholesterol | apoB-48 | ApoE-dependent hepatic clearance; atherogenic remnant particle |
| VLDL | 0.930–1.006 | 30–80 nm | Triglyceride | apoB-100 | Carries hepatic triglyceride; produces fasting turbidity when increased |
| Intermediate-density lipoprotein (IDL) | 1.006–1.019 | 25–35 nm | Triglyceride and cholesterol | apoB-100 | VLDL remnant that is cleared by the liver or converted to LDL |
| Low-density lipoprotein (LDL) | 1.019–1.063 | 18–25 nm | Cholesterol | apoB-100 | Major cholesterol-delivery particle and atherogenic particle |
| High-density lipoprotein (HDL) | 1.063–1.210 | 5–12 nm | Cholesterol and phospholipid | apoA-I | Heterogeneous particles involved in cholesterol efflux and transport to the liver |
| Lipoprotein(a), Lp(a) | About 1.055–1.085 | About 30 nm | Cholesterol | apoB-100 plus apo(a) | Genetically influenced, atherogenic LDL-like particle |
Apolipoprotein functions
Apolipoproteins stabilize particles, direct receptor binding, and regulate enzymes. Exchangeable apolipoproteins move among particles. Each chylomicron carries one apoB-48 molecule, and each VLDL, IDL, LDL, or Lp(a) particle carries one apoB-100 molecule. Plasma apoB concentration therefore approximates the number of circulating apoB-containing particles. LDL particle number is a separate measurement.2
| Apolipoprotein | Main particles | Laboratory-relevant function |
|---|---|---|
| apoA-I | HDL; some chylomicrons | Major HDL structural protein; activates lecithin-cholesterol acyltransferase (LCAT) |
| apoA-II and apoA-IV | HDL; chylomicrons | Structural and metabolic roles in HDL and intestinal lipid transport |
| apoB-48 | Chylomicrons and remnants | Intestinal structural protein produced by RNA editing of APOB messenger RNA |
| apoB-100 | VLDL, IDL, LDL, Lp(a) | Hepatic structural protein and LDL-receptor ligand |
| apoC-II | Chylomicrons, VLDL, HDL | Cofactor for lipoprotein lipase (LPL) |
| apoC-III | Chylomicrons, VLDL, HDL | Inhibits LPL-mediated lipolysis and remnant uptake |
| apoE | Remnants, IDL, some HDL | Ligand for hepatic remnant and LDL receptors; apoE2 binds the LDL receptor poorly |
| apo(a) | Lp(a) | Plasminogen-like protein joined to apoB-100 by a disulfide bond |
Lp(a) concentration is largely inherited. Apo(a) size varies with the number of kringle IV type 2 repeats, which complicates mass-based measurement. Laboratories should identify the assay and report its calibrated unit. A fixed conversion between mg/dL and nmol/L is invalid because particle mass varies with apo(a) size. The 2026 ACC/AHA multisociety guideline recommends at least one Lp(a) measurement for every adult and classifies a concentration of 125 nmol/L or 50 mg/dL as high. A concentration of 250 nmol/L or about 100 mg/dL is associated with approximately twice the long-term atherosclerotic cardiovascular disease (ASCVD) risk of the population median. These mass and molar pairings are approximate.3
Lipoprotein transport
Short- and medium-chain fatty acids can pass directly into portal blood. Enterocytes re-esterify long-chain fatty acids and package them with apoB-48 into chylomicrons. Three connected pathways then move lipid among the intestine, liver, peripheral tissues, and plasma.1,2
- Exogenous pathway. Chylomicrons enter blood through intestinal lymph. LPL hydrolyzes their triglyceride in muscle and adipose capillaries. ApoC-II activates this reaction, while apoC-III inhibits it. The smaller, cholesterol-enriched remnants return to the liver through apoE-dependent receptor uptake.
- Endogenous pathway. The liver secretes triglyceride-rich VLDL with apoB-100. LPL removes triglyceride and produces IDL. The liver clears some IDL through apoE-mediated uptake; hepatic lipase converts the remainder toward LDL. LDL receptors recognize apoB-100 and clear most LDL in the liver. Rising intracellular cholesterol reduces LDL-receptor and HMG-CoA-reductase activity.
- Reverse cholesterol transport. Lipid-poor apoA-I accepts cell phospholipid and cholesterol through the ATP-binding cassette transporter A1 (ABCA1). LCAT esterifies free cholesterol and moves the ester into the HDL core. HDL delivers cholesterol to the liver through scavenger receptor class B type I or transfers cholesteryl ester to apoB particles through cholesteryl ester transfer protein (CETP). HDL particles remain heterogeneous throughout this remodeling.
Proprotein convertase subtilisin/kexin type 9 (PCSK9) directs LDL receptors toward lysosomal degradation. Pathogenic variants that reduce LDL-receptor function, impair apoB-100 binding, or increase PCSK9 activity can produce familial hypercholesterolemia (FH). The resulting decrease in LDL clearance causes lifelong elevation of LDL cholesterol (LDL-C).
Specimens and routine measurement
Use the serum or plasma type validated by the assay. Document fasting status at collection and report it with the lipid profile. A fasting or nonfasting lipid profile is acceptable for routine assessment. The 2026 ACC/AHA guideline calls for a fasting profile when a nonfasting triglyceride result is at least 400 mg/dL (4.5 mmol/L), when a triglyceride-metabolism disorder is known or suspected, or when family history raises concern for inherited dyslipidemia.3,4
| Analyte | Common routine principle | Main limitations |
|---|---|---|
| Total cholesterol | Cholesteryl ester hydrolase releases cholesterol; cholesterol oxidase forms hydrogen peroxide; peroxidase produces a colored dye | Bilirubin and ascorbate can consume peroxide and lower results in susceptible methods |
| Triglycerides | Lipase releases glycerol; glycerol kinase and glycerol-phosphate oxidase form hydrogen peroxide for a color reaction | Free glycerol causes a method-dependent positive bias; the effect is usually small, while uncommon marked glycerolemia can produce pseudohypertriglyceridemia |
| HDL-C | Precipitation removes apoB particles before cholesterol measurement, or a homogeneous reagent selectively measures HDL-associated cholesterol | Selectivity can change in severe dyslipidemia and unusual lipoprotein composition |
| LDL-C | Estimated from a standard lipid profile, measured by beta-quantification, or measured by a homogeneous assay | Calculation and direct-assay limitations depend on triglyceride concentration and abnormal particles |
| apoB, apoA-I, Lp(a) | Immunoturbidimetric or immunonephelometric measurement | Calibration, antibody specificity, and particle heterogeneity affect method comparison; Lp(a) also requires attention to apo(a) isoform sensitivity and reporting units |
Glycerol blanking measures free glycerol separately before triglyceride calculation. It is useful when the result and specimen appearance disagree or pseudohypertriglyceridemia is suspected. Triglycerides also have substantial within-person biological variation, so a serial change reflects both biological and analytical variation.4
LDL-C and non-HDL-C
Beta-quantification is the reference approach for LDL-C. Ultracentrifugation at density 1.006 g/mL removes chylomicrons and VLDL. LDL-C is then obtained from cholesterol in the infranatant after accounting for HDL-C. Its time, specimen-volume, and equipment requirements limit routine use.4
The Friedewald equation estimates VLDL cholesterol from triglycerides and subtracts it with HDL-C from total cholesterol. Its original validation used fasting specimens, and it becomes invalid at triglycerides of 400 mg/dL (4.5 mmol/L) or higher. Accuracy also declines as triglycerides rise and LDL-C falls. Chylomicrons, familial dysbetalipoproteinemia, and lipoprotein X (LpX) can invalidate the assumptions. Current guidance prefers Martin/Hopkins or Sampson/NIH estimation for routine calculated LDL-C and prefers these estimates over routine direct LDL-C assays when they apply.3,4,5
The equations and a worked example appear in Core Chemistry Calculations.
Non-HDL-C = total cholesterol − HDL-C. It includes cholesterol in LDL, IDL, VLDL, Lp(a), and chylomicron-remnant particles. The calculation uses two measured values, works with fasting and nonfasting specimens, and is particularly useful when triglycerides are elevated. Current guidance recommends reporting non-HDL-C with the standard lipid profile.3
LpX is a phospholipid- and free-cholesterol-rich particle associated most strongly with cholestasis and LCAT deficiency. It lacks apoB. Calculated LDL-C can assign LpX cholesterol to the LDL fraction, and homogeneous LDL-C assays show method-dependent positive or negative reactivity. A large LDL-C result accompanied by low apoB and cholestatic findings should prompt review for LpX. Electrophoresis with specialized staining can support characterization where the laboratory has a validated procedure.4,6
Standardization and comparability
Clinical decision points depend on comparable results across methods. The Centers for Disease Control and Prevention (CDC) Cardiovascular Disease Reference Laboratory uses ultracentrifugation reference procedures for LDL-C and HDL-C, Abell-Kendall and mass-spectrometry-based procedures for total cholesterol, and mass spectrometry for total glycerides. These procedures are calibrated to National Institute of Standards and Technology reference materials.7
The CDC Cholesterol Reference Method Laboratory Network transfers this accuracy base to manufacturers and laboratories through comparisons with fresh, unmodified serum. Fresh patient samples matter because lyophilization, spiking, and other processing can change the way a material reacts in a routine assay. A material that behaves like patient samples across methods is commutable.8
Lipid standardization programs use performance goals of approximately 3% imprecision and 3% bias for total cholesterol, 4% imprecision and 5% bias for HDL-C, 4% imprecision and 4% bias for LDL-C, and 5% imprecision and 5% bias for triglycerides. The method’s stated limits and the laboratory’s verification data govern patient testing.4
Decision points
Adult lipid results are interpreted with clinical decision limits and ASCVD risk. A universal adult reference interval would hide the large effect of prior ASCVD, age, diabetes, chronic kidney disease, and other risk factors. The 2026 ACC/AHA guideline uses PREVENT-ASCVD equations for primary prevention in adults aged 30 to 79 years with LDL-C from 70 to 189 mg/dL and no known ASCVD or subclinical atherosclerosis.3
| PREVENT-ASCVD 10-year risk | Category | LDL-C goal used when lipid-lowering therapy is started | Corresponding non-HDL-C goal |
|---|---|---|---|
| 3% to <5% | Borderline | <100 mg/dL | <130 mg/dL |
| 5% to <10% | Intermediate | <100 mg/dL | <130 mg/dL |
| ≥10% | High | <70 mg/dL | <100 mg/dL |
| Clinical ASCVD at very high risk | Secondary-prevention category | <55 mg/dL | <85 mg/dL |
For a 10-year risk below 3%, the guideline emphasizes health-behavior counseling. Drug therapy can still be considered when another indication is present, including LDL-C of 160–189 mg/dL or a 30-year ASCVD risk of at least 10%.3
Pediatric interpretation uses age-specific decision limits. The guideline-based cut points below identify acceptable, borderline, and abnormal results; they are separate from adult risk-based goals.3
| Analyte | Acceptable | Borderline | Abnormal |
|---|---|---|---|
| Total cholesterol | <170 mg/dL | 170–199 mg/dL | ≥200 mg/dL |
| LDL-C | <110 mg/dL | 110–129 mg/dL | ≥130 mg/dL |
| Non-HDL-C | <120 mg/dL | 120–144 mg/dL | ≥145 mg/dL |
| Triglycerides, age 0–9 years | <75 mg/dL | 75–99 mg/dL | ≥100 mg/dL |
| Triglycerides, age 10–19 years | <90 mg/dL | 90–129 mg/dL | ≥130 mg/dL |
| HDL-C | >45 mg/dL | 40–45 mg/dL | <40 mg/dL |
Dyslipidemia patterns
Retention of apoB-containing particles in the arterial wall promotes oxidation, macrophage uptake, foam-cell formation, and atherosclerotic plaque. Secondary causes can produce or amplify a lipid pattern. Diabetes, hypothyroidism, kidney disease, cholestasis, alcohol exposure, obesity, and selected medications are common considerations. Correlate the profile with fasting status, specimen appearance, medications, organ-function tests, and family history.
Fredrickson and WHO phenotypes
The Fredrickson and World Health Organization system classifies the elevated particle phenotype. It remains useful for recognizing patterns, while current etiologic diagnosis uses clinical, biochemical, and genetic evidence.9
| Type | Main increased particle | Standing plasma appearance | Typical lipid pattern | Main association |
|---|---|---|---|---|
| I | Chylomicrons | Cream layer over clear plasma | Triglycerides markedly increased | LPL-pathway defect, including LPL or apoC-II deficiency |
| IIa | LDL | Clear | Total cholesterol and LDL-C increased; triglycerides within the expected range | FH or polygenic hypercholesterolemia |
| IIb | LDL and VLDL | Clear to turbid | Cholesterol and triglycerides increased | Familial combined hyperlipidemia and other mixed patterns |
| III | Remnants, historically called beta-VLDL | Turbid | Cholesterol and triglycerides increased, often to similar magnitudes | Familial dysbetalipoproteinemia |
| IV | VLDL | Turbid without a cream layer | Triglycerides increased | Insulin resistance, diabetes, alcohol exposure, or familial VLDL excess |
| V | Chylomicrons and VLDL | Cream layer over turbid plasma | Triglycerides markedly increased; cholesterol also increased | Mixed chylomicron and VLDL accumulation |
Characteristic disorders
Familial hypercholesterolemia. Heterozygous pathogenic variants most often affect LDLR and less often APOB or PCSK9. LDLR loss of function reduces receptor-mediated clearance, selected APOB variants impair LDL binding, and PCSK9 gain of function accelerates receptor degradation. The laboratory pattern is lifelong, often marked LDL-C elevation with triglycerides near the expected range. Biallelic disease has a more severe phenotype, and untreated LDL-C may exceed 500 mg/dL. Molecular testing can establish the genetic cause; lipid testing of relatives supports cascade detection.10
Familial combined hyperlipidemia. Increased hepatic production of apoB-containing particles can produce high LDL-C, high triglycerides, or both among members of one family. ApoB can remain high when LDL-C alone appears less striking.11
Familial dysbetalipoproteinemia. Impaired apoE-mediated remnant clearance produces cholesterol-rich VLDL and chylomicron remnants. ApoE2/E2 is the usual susceptibility genotype, and expression varies with secondary metabolic factors. A VLDL-C to total-triglyceride ratio above 0.30 when both values use mg/dL is a historical clue derived from ultracentrifugation. Current evaluation combines the lipid phenotype with apoB-based ratios and compatible APOE testing.12
Hypertriglyceridemia. The 2026 guideline defines elevated triglycerides as at least 150 mg/dL fasting or at least 175 mg/dL nonfasting. A concentration of at least 500 mg/dL is severe. Chylomicrons become prominent as concentrations rise, and pancreatitis risk becomes a major concern at 1,000 mg/dL (11.3 mmol/L) or higher. Evaluate secondary causes and inherited triglyceride-clearance disorders when the result is severe or persistent.3
Low apoB-containing lipoproteins. Heterozygous APOB-related familial hypobetalipoproteinemia usually produces low total cholesterol, LDL-C, and apoB with few symptoms; hepatic steatosis may be present. Biallelic disease can cause severe fat malabsorption, fat-soluble-vitamin deficiency, and neurologic or retinal findings.13 Abetalipoproteinemia from biallelic MTTP variants prevents secretion of apoB-containing lipoproteins. LDL-C and apoB are absent or extremely low, triglycerides are very low, and acanthocytes and fat-soluble-vitamin deficiencies support the pattern.14
Severe HDL deficiency (hypoalphalipoproteinemia). Loss-of-function ABCA1 variants cause Tangier disease. HDL-C is typically below 5 mg/dL, and apoA-I is usually below 30 mg/dL and often below 5 mg/dL because cells cannot transfer cholesterol and phospholipid normally to apoA-I. Tissue cholesteryl-ester accumulation, orange tonsils, neuropathy, and hepatosplenomegaly can accompany the lipid pattern. Other causes of very low HDL-C include apoA-I and LCAT defects.15
References
- Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
- Feingold KR. Introduction to lipids and lipoproteins. Updated January 14, 2024. In: Feingold KR, Adler RA, Ahmed SF, et al, eds. Endotext. MDText.com, Inc; 2000-. Accessed August 29, 2026. NCBI Bookshelf.
- Blumenthal RS, Morris PB, Gaudino M, et al. 2026 ACC/
AHA/ AACVPR/ ABC/ ACPM/ ADA/ AGS/ APhA/ ASPC/ NLA/ PCNA guideline on the management of dyslipidemia: a report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. Circulation. 2026;153:e1154-e1276. doi:10.1161/CIR.0000000000001423. - Cao J, Donato LJ, El-Khoury JM, et al. ADLM guidance document on the measurement and reporting of lipids and lipoproteins. J Appl Lab Med. 2024;9(5):1040-1056. doi:10.1093/jalm/jfae057.
- Friedewald WT, Levy RI, Fredrickson DS. Estimation of the concentration of low-density lipoprotein cholesterol in plasma, without use of the preparative ultracentrifuge. Clin Chem. 1972;18(6):499-502. doi:10.1093/clinchem/18.6.499.
- Matsushima K, Sugiuchi H, Anraku K, et al. Differences in reaction specificity toward lipoprotein X and abnormal LDL among 6 homogeneous assays for LDL-cholesterol. Clin Chim Acta. 2015;439:29-37. doi:10.1016/j.cca.2014.09.030.
- Centers for Disease Control and Prevention. CVD reference laboratory. Updated April 24, 2024. Accessed August 29, 2026. CDC CVD Reference Laboratory.
- Centers for Disease Control and Prevention. Improving performance: Cholesterol Reference Method Laboratory Network. Updated December 2, 2025. Accessed August 29, 2026. CDC Cholesterol Reference Method Laboratory Network.
- Beaumont JL, Carlson LA, Cooper GR, Fejfar Z, Fredrickson DS, Strasser T. Classification of hyperlipidaemias and hyperlipoproteinaemias. Bull World Health Organ. 1970;43(6):891-915. PMID:4930042.
- Ison HE, Clarke SL, Knowles JW. Familial hypercholesterolemia. Updated January 30, 2025. In: Adam MP, Bick S, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993-2026. Accessed August 29, 2026. NCBI Bookshelf.
- Patni N, Ahmad Z, Wilson DP. Genetics and dyslipidemia. Updated April 17, 2023. In: Feingold KR, Adler RA, Ahmed SF, et al, eds. Endotext. MDText.com, Inc; 2000-. Accessed August 29, 2026. NCBI Bookshelf.
- Bea AM, Cenarro A, Marco-Benedí V, et al. Diagnosis of familial dysbetalipoproteinemia based on the lipid abnormalities driven by APOE2/E2 genotype. Clin Chem. 2023;69(2):140-148. doi:10.1093/clinchem/hvac213.
- Burnett JR, Hooper AJ, Hegele RA. APOB-related familial hypobetalipoproteinemia. Updated September 9, 2021. In: Adam MP, Bick S, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993-2026. Accessed August 29, 2026. NCBI Bookshelf.
- Burnett JR, Hooper AJ, Hegele RA. Abetalipoproteinemia. Updated May 19, 2022. In: Adam MP, Bick S, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993-2026. Accessed August 29, 2026. NCBI Bookshelf.
- Burnett JR, Hooper AJ, McCormick SPA, Hegele RA. Tangier disease. Published November 21, 2019. In: Adam MP, Bick S, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993-2026. Accessed August 29, 2026. NCBI Bookshelf.